The present invention relates generally to reactions of aromatic compounds. In particular, it relates to methods of carrying out electrophillic substitution reactions on aromatic compounds using microreactors.
Aromatic compounds undergo a number of electrophillic substitution reactions, such as nitration and sulphonation, using a variety of reagents. As an example, aromatic compounds can be nitrated through the use of nitric acid and a catalyst such as sulphuric acid, which are commonly brought into contact with the organic compound to be nitrated in a reactor vessel. The product, a nitroaromatic, then has to be separated from the resulting mixture using some suitable means such as solvent extraction or distillation and the aqueous phase recycled. Such separation procedures add considerable cost and complexity to the process. In addition, undesired by-products may be produced in the reaction, e.g. dinitrobenzene in the formation of nitrobenzene. These by-products may result in further purification of the product.
Numerous micropreparative and microanalytical methods, and corresponding equipment, are available to the chemist. For example, D1: L. MÉSZÀROS and 1. MÉSZÀROS: xe2x80x98Kontinuerlich arbeitende Fadenreaktoren fxc3xcr mikroprxc3xa4parative Zweckexe2x80x99 FETTE, SEIFEN, ANSTRICHMITTEL., vol. 70, no. 12, 1968, pages 940-941, XP002095576 discloses a thread reactor and its use in the preparation of dinitrobenzene and the sulfonation of decylbenzene. The reactants are fed down two glass threads which are brought together to a single thread where the reactants mix and form an emulsion without any mechanical intervention.
According to the present invention there is provided a method of reacting an aromatic compound with a reacting agent, the method comprising providing a first flow path for the aromatic compound and a second flow path for a reacting agent, the reacting agent being immiscible with the aromatic compound and the flow paths communicating with each other in a region in which the aromatic compound and the reacting agent can contact one another, flowing the aromatic compound and the reacting agent through the first and second flow paths respectively such that, at least in the said region, the flow of the aromatic compound and the reacting agent is essentially laminar, and a stable open interface is formed therebetween, at least the first flow path in the interface region having a width perpendicular to the interface in the range 10-1,000 micrometres, allowing at least a portion of the aromatic compound to react with the reacting agent and flowing the reacted aromatic compound and the reacting agent away from said region, the reaction being carried out without substantial mixing of the unreacted aromatic compound and the reacting agent.
It has been found that the use of a so-called xe2x80x98microreactorxe2x80x99, that is a reactor having a flow path dimension perpendicular to the interface of the two liquid phases of less than 1,000 micrometres, according to the present method, for the nitration of aromatic compounds provides unexpected improvements in process control including significant improvements in both reaction product yield and purity.
The present method also has advantages over conventional methods, in producing an organic product stream which requires no separation from the aqueous reactants and products.
The flow rates of the reactants can also be balanced such that a stoichiometric reaction occurs, thereby resulting in a more efficient and cost-effective process which leaves little or no unreacted reagents which would otherwise reduce the yield of the main product. This also reduces the need for extensive purification procedures for the product.
The flow path carrying the aromatic compound may have a width (defined as perpendicular to the liquid-liquid interface) in the range 10-1,000 micrometres. Preferably, the width lies in the range 30-300 micrometres. Most preferably, the width lies in the range 50-150 micrometres.
The length of the interface region (measured in the direction of the flow) may typically lie in the range 10 mm to 1 metre. For example, a reactor length of 10 centimetres has been used to produce high yields. The optimum reactor length for a particular reaction will be dependant on the flow rates and reaction kinetics in each case.
Typically, the microreactor used in the present method is the same general type of apparatus as disclosed in patent applications WO 96/12541 and WO 96/12540 and the teaching of those documents is incorporated herein by reference.
Patent applications WO 96/12541 and WO 96/12540 disclose the advantages of using microengineered fluid flow paths primarily in solvent extraction processes. Surprisingly, we have found that using the apparatus described in WO 96/12541 and WO 96/12540 to carry out aromatic nitration reactions provides unexpectedly large improvements in both product yield and purity.
The improvements in reaction control provided by the present method are thought to arise from a number of features.
The reacting medium has a high surface area to volume ratio which is thought to allow very efficient heat dissipation to the walls of the reactor. In the case of exothermic reactions, heat generated by the reaction will be carried away from the reacting medium thus reducing the tendency of side products to form. Conversely, the high surface area to volume ratio may also allow efficient transfer of heat into the reacting medium from external sources as required. Thus the microreactor provides an efficient means for heat sinking from or heat sourcing to the fluid reacting region. The high surface area to volume ratio also provides for a high interfacial area for chemical transfer compared with the volume of fluid to be reacted.
The small width of the flow path means that reacting species diffuse over much shorter distances, particularly over distances associated with the diffusion boundary layer width, before they finally react with other reagents than in conventional reactors.
The use of a flow path with a width perpendicular to the liquid interface ranging from 10 to 1,000 micrometres allows very accurate control over very low flow rates. This fine control over flow rate together with precise control over residence time in the reactor provides a highly controllable reacting system which may enable highly reactive intermediate products to be formed in high yield. Such highly reactive intermediates can be difficult to produce under conventional reacting conditions and so may be very valuable. The intermediate may be used in further reactions. The intermediate may be removed from the reactor, or additionally or alternatively, the reaction may be halted before reaching the final product by quenching it with a heat sink or through other methods such as the use of suitable reagents.
The fine fluidic control of the present method also has the advantage of enabling the matching of the input reagents to the correct stoichiometry of the reaction. This can result in a more efficient and cost-effective process which leaves little or no unreacted reagents which would otherwise reduce the yield of the main product. This also reduces the need for extensive purification procedures for the product.
The nitration reaction involves reaction of a first phase comprising an organic aromatic compound on a second phase comprising a nitrating agent to produce two new phases of different chemical composition to the starting phases. The aqueous and organic phases produced are ideally separated such that minimum contamination occurs.
Typically, the nitrating agent is a mixture of nitric acid and sulphuric acid. The reaction is preferably carried out at elevated temperatures, for example, in the nitration of benzene, at 60xc2x0-140xc2x0 C., preferably at 90xc2x0-120xc2x0 C.
The mass concentration of sulphuric acid in the sulphuric acid/nitric acid mixture is typically from 60%-85%, preferably 65%-80% more preferably from 70%-75%. The mass concentration of nitric acid is preferably from 3%-5%.
The organic volume is preferably from 5%-20% of the total and more preferably of the order of 10%.
As indicated above, a preferred reactor length for this nitration reaction is in the range 50 xcexcm to 150 xcexcm, such a length taking into account reaction performance on the one hand and pressure drop and blockage factors on the other hand.
Preferred conditions for nitration of aromatic compounds include a sulphuric acid range of 70-75%, about 3% nitric acid, about 10% volumetric organic flow and a temperature of about 100xc2x0 C. In general the nitric acid content should be balanced with the organic content. Below 5% organic may result in instability and above 20% organic may require excessive quantities of nitric acid, thereby possibly causing DNP to increase and the strength of the sulphuric acid to fall to too low a value.
Other examples of this type of reaction include the sulphonation of an aromatic compound using sulphuric acid as the sulphonating agent. The aromatic compound is slowly consumed in the reaction yielding a single aqueous phase. Reactions of the type with which this invention is concerned may be enhanced by virtue of the short diffusion distances over which the reagents must diffuse. Such diffusion distances are characterised by the expression Dt/12 where D is the diffusion coefficient, t is the time taken for transport of the reagent before it reacts with the other reagents and 1 is the length scale over which diffusion takes place. For substantial transport (50-100%) of the catalysed reagent, Dt/12 lies in the range 0.1-1 (see J. Crankxe2x80x94The Mathematics of Diffusionxe2x80x94Second Editionxe2x80x94Oxford University Press, 1975). Typical values of D for liquids lie between 10xe2x88x9210-10xe2x88x929 m2/s which, for transport times of around 1 second, require length scales and thus reactor dimensions normal to the reactor surface of between 30-100 microns.
The improved reaction control in the present method allows the production of reagents under highly defined conditions. This control will allow hazardous reagents to be produced and controlled such that they are maintained in a safe manner. The reduced inventory of the reagents, both within the lead-in flow paths or microchannels and within the microreactor itself reduces potential risks associated with handling hazardous or explosive reagents.
When large quantities of fluid are required to be reacted, such as in many practical embodiments, a large number of microreactors may be employed. Since large numbers of microreactors may be manufactured relatively cheaply, this provides an efficient way of reacting large quantities of fluid under highly controlled conditions. In addition, in such axe2x80x9cscale-upxe2x80x9d, the reaction conditions in the microreactors, and hence product distribution, remain unchanged. This is an advantage in comparison to conventional batch reactors where the distribution of products may change as the reaction is scaled up from laboratory-scale to plant-scale.